The intention of this invention is to increase the flexibility of implementation of knuckle boom systems utilizing knuckle boom hydraulic circuits described in Canadian Patent 2,317,670.
The aforementioned patents aim to achieve horizontal reaching with greater energy efficiency and greater operator ease. This is achieved by hydraulically transferring gravitational potential energy between the cylinders supporting the main boom and the stick boom and creating near horizontal movement with 1 operator control. Another cylinder is mechanically connected either like the stick cylinder or like the hoist cylinder and is hydraulically controlled independently of the hoist and the stick cylinder. This 3rd cylinder controls the geometric relationship between the main boom and the stick boom.
During normal operation, the implement is held above the ground and the main boom and the stick boom must support the weight of the implement and payload. The hydraulic port of the cylinder that must be pressurized to support the load (with the knuckle boom system in the far reach position) will be referred to as the lifting end of the cylinder. The opposite end of the cylinder will be referred to as the lowering end of the cylinder. (However, there are some booms whose geometry cause 1 or both cylinder to reverse while traversing from the far reach position to close reach position).
The lifting ends of the cylinders are connected hydraulically together to allow transferring of potential energy from one cylinder to the other. For a feller buncher, typically the lifting ends of the cylinders are the base ends of the cylinders.
To reduce the amount of oil flowing to and from tank when the third cylinder is stroked while the hydraulic controls for the stick and hoist are not actuated, the lowering ends of hoist and stick are connected hydraulically in a similar manner, as were the lifting ends.
While most of the time, the lifting ends of the cylinders are pressurized; there are situations where the load on the cylinders must reverse direction. This typically occurs when pushing the implement into the ground or pulling backwards towards the carrier.
To allow for reversing of the cylinder loads and to allow the 3rd cylinder to work independently of the hydraulic control of the hoist and stick cylinder, the oil flow out of the lowering ends of the hoist and stick cylinders must be controlled wisely.
This can be accomplished in several ways:
First, eliminate the fluid exchange to and from the hydraulic tank by matching the area ratio of the lowering ends and the lifting ends between the hoist cylinder and the stick cylinder.
The following equation must be true:             AREA_Hoist      LIFTING              AREA_Hoist      LOWER        =            AREA_Stick      LIFTING              AREA_Stick      LOWER      
Second, have the operator manually activate a valve during these motions. If the area ratios are unequal.
Third automatically open a valve to allow the fluid out during these motions. If the area ratios are unequal.
The first option requires keeping the area ratios between the hoist and stick cylinders equal. However, the requirements of the hoist cylinder and stick cylinder are often very different. The volume of fluid in the lifting end of the hoist cylinder tends to be greater than the volume of fluid in the lifting end of the stick cylinder. This would necessitate using cylinders with exactly the same bore and rod sizes and different strokes, using specifically sized cylinders with the same bore to rod ratio, but with different bores and rod sizes or using multiple cylinders in parallel that have the same bore and rod size. Also, hoist and stick cylinders would have to have the same orientation. With both cylinders either pushing or pulling. If the above requirements cannot be met, then there will be a net change in the volume of the lowering ends when the third cylinder is activated alone.
If the bore/rod ratios of the hoist and stick cylinder cannot be made equal, then there will be an exchange of fluid between the lowering ends and the reservoir when the third cylinder is stroked by itself.
Hence the second and third options allow greater flexibility in the selection of cylinders sizes and orientation of the hoist cylinder and the stick cylinder. However, this flexibility requires managing the exchange of fluid from the lowering ends when the third cylinder is stroked. The third option doesn""t require special operator intervention, but may require some non-intrusive intervention by operation.
Even if the area ratios are maintained, there may be times where transient exchanges of fluid will be required between the lowering ends and the reservoir. This can occur for instance when stroking near the cushions of the cylinders.
A hydraulic valve is used to judiciously shunt oil from lowering ends of the hoist and stick cylinders that have been plumbed to achieve near horizontal movement. This allows the use of cylinders that have unequal rod/bore ratios or to compensate for certain transient effects that may require the exchange of oil to the reservoir. The action of the hydraulic valve acts without obtrusive operator intervention and is essentially automatic. This allows for greater flexibility in the design of the boom system, allows the boom system to perform various tasks, while not requiring obtrusive intervention from the operator.